Table of Contents
Abstract……………………………………………………….3
Nomenclature…………………………………….……………3
- Introduction………………………………..……….……4
- CFD Analysis Roadmap…………………..…….………4
- Theory……………………………….….…..…….7
- Module Example—Laminar FlowPasta Cylindrical Pipe……… 18
- Summary of Tutorials……………………………….…….. 27
- Conclusion ………………………………………..……. 43
- References………………………………………………. 44
- Appendices- Modules
A--Laminar Pipe Flow……………………….……………
B--Turbulent Pipe Flow…………………….………..……
C--Laminar Flow Over Flat Plate (Geometry and Mesh)…
D--Laminar Flow Over Flat Plate…………………………
E--Nozzle Tutorial ………………………………………..
F--Jets: Turbulent Flow……………………………..…….
G—External Turbulent Compressible and Incompressible Flow across an airfoil…. `
H—Turbulent Incompressible Flow across a Periodic Airfoil …………
I—Discrete Phase Modeling: Particle Injection into a Pipe………………
Abstract
ANSYS FLUENT, computational fluid dynamics (CFD) software is very difficult to use especially for new users. To help with this, tutorials have been created to be able to guide users in the use of ANSYS FLUENT. They were created to mimic a classroom-like structure, where the fundamentals are learned first. The creation of fundamental tutorials will allow users to have projects where they will utilize the learning modules as references to guide them in more complicated projects. Through the use of FLUENT and several validation efforts, which are referenced from scholarly sources, the user will be able to validate the accuracy of their results. In addition, with the help of the provided learning modules, the user will be able to create a roadmap to achieve competence to solve more complicated problems.
Nomenclature
AArea
Coefficient of friction along wall
Coefficient of Lift
Coefficient of Drag
Coefficient of Pressure
DDiameter
Gravity
-Turbulence model to simulate and read turbulent flow
M Mach number
PPressure
ReReynolds number
Reynolds number along a position x
Centerline Velocity
Fluid velocity
uFriction velocity
Max. Velocity
VVelocity
Inlet Velocity
YpDistance to the wall from center of pipe
Non-dimensionalizeddistance of first grid point from wall
– Density
–Shear Stress
–Dynamic viscosity
- Introduction
One of the biggest challenges in the engineering industry is being able to come up with efficient and optimal designs for new products. One of the strongest tools offered is FLUENT. FLUENT is a very useful program recently acquired by ANSYS. It has the capability to model fluid flow past objects with the ability to design, test, and analyze results all under one program. Although it is a strong tool for engineering, it is also very difficult to use. For this reason, tutorials have been created to teach FLUENT with the hope that these tutorials will serve as a fundamental tool in teaching and as references for future senior design projects. The way the tutorials are set up are by creating and analyzing basic flow fields and then to ensure the accuracy of each test case it is then validated against a scholarly reference.
The structure of the tutorials is to first reproduce the fundamentals learned in a Fluid Mechanics and Thermo Dynamics courses. One of the first scenarios learned in fluid mechanics is the flow through a cylindrical pipe. The tutorials created follow very closely to how a fluid mechanics course would be taught. For this reason, the first tutorial is the laminar flow of fluid through a cylindrical pipe. The next tutorial is turbulent flow of fluid through a cylindrical pipe. By doing the turbulent case, it will allow the user to see the difference between laminar and turbulent flows and to gain some insight as to why different methods of analyzing structures in FLUENT are necessary.
The next created learning module is to analyze flow over a flat plate. Analyzing the flow over a flat plate is very important because it will give the user a more in-depth look as to what happens when flow passes over an object. In addition, other tutorials such as a turbulent flow past a nozzle, turbulent jet flow, turbulent compressible and incompressible flow past an airfoil, turbulent incompressible flow past a periodic airfoil, and a discrete phase modeling tutorials are created to be able to serve as fundamental tutorials so that the user may then use them as precursors to analyzing more complicated problems.
In addition, one of the most important parts in creating the tutorials is the need for validation. Validation is extremely important when analyzing solutions, because it is the only way to ensure the accuracy of the results obtained in FLUENT. Validation is made by comparing results from FLUENT to theoretical and experimental data from scholarly sources.
- CFD Analysis Roadmap
The importance of the created learning tutorials is to guide users into ANSYS FLUENT and provide them with a friendly introduction to the CFD software. For this reason fundamental learning modules have been created which are: laminar and turbulent fluid flow through a cylindrical pipe, laminar fluid flow over a flat plate, turbulent flow through a nozzle, turbulent jet flow, turbulent compressible and incompressible flow past an airfoil, turbulent incompressible flow past a periodic airfoiland discrete phase modeling. So why were these specific modules chosen and created?
- The purpose for these tutorials is to lead a new user through options of increasing difficulty. The laminar pipe flow tutorial helped to introduce the icons and tools that ANSYS has to offer. By having the user work with this very simple tutorial, they could familiarize themselves with where certain icons are and where certain tools are located. Once the user has completed this tutorial then the next tutorial added increased in complexity. The reason for this method is to be able to instill confidence in the user to make them feel confident in doing simple cases and build up to more complicated ones.
- After each of the simple cases is run, the user has to validate each result. By validating one’s results the user is ensured they have created an accurate simulation. For instance, if a first time user has to analyze an airfoil, they should not start by designing an airfoil. Although they might obtain results, how would they know if the results are accurate? For this reason, the user would first figure out how flow develops through a pipe.
- The user would take the laminar tutorial and figure out how to model fluid flow and then be able to validate it. Next, since the airfoil is close to flow over an isolated surface, they would then want to analyze the flat plate flow tutorial. Again the user should then have to validate these results.
- Next, since the airfoil is going to have a specific set of coordinates, the user could then want to use the Nozzle tutorial, which explains how to import coordinates in order to create an object. Now the user is ready to create an airfoil and analyze it. The user now knows how to model flow and initialize a solution (laminar tutorial), they also know how to model flow over an isolated surface (flat plate tutorial) and they know how to import coordinates into FLUENT (nozzle tutorial). Since they have all the information needed to create an accurate airfoil, they user can now apply the previous knowledge to analyze a complicated geometry. From here the user is a step closer to creating an accurate airfoil and of course like all the other tutorials the user needs to validate the results. Refer to Fig.1 for a visual representation of the mentioned roadmap.
Fig.1—Example of a Roadmap
- Another example of a roadmap is for analyzing the flow through a guide vane. First the novice user will want to be able to model fluid flow so they would begin with a laminar tutorial. Once they have learned the icons and what each tool does, then they would want to analyze turbulent flow. They would then refer to the turbulent flow through a pipe and figure out how to apply biasing and what models to use to analyze turbulence (See appendix B). The user will then want to again create a 2D airfoil which would give them the knowledge to analyze how to fluid passes an isolated surface. Once they have validated this they can then move to a 3D airfoil and again validate it. Then they would want create a 2d cascade, validate it, and lastly create a 3D cascade and validate it. Now the user has the necessary information and knowledge to create a guide vane and have the confidence to know that it is accurate. Refer to Fig.2 for a visual representation of the described roadmap.
Fig.2—Example of a Guide Vane Roadmap
The creation of a roadmap is of crucial importance in order to be able to build the knowledge on how to create and analyze complicated geometry. Before wanting to analyze any complicated geometry, the user should make a roadmap of their own so that they can build confidence in how they will figure out the problem and make sure it will be accurate. Like mentioned before the user can create any geometry they want and can get results but how will they know if it is accurate? The only way of knowing this is by simplifying the complicated object into several steps (the roadmap) and work part by part in order to have accurate analytical data for their object. Sometimes validation for complex problems is not readily available. If however a roadmap is correctly followed then the need for validation for the specific complex problem in question while needed is not as crucial.
- Theory
ANSYS FLUENT is Computational Fluid Dynamics (CFD) software that allows users to simulate flow problems of ranging complexity. It contains broad physical modeling capabilities needed to model flow, turbulence, heat transfer, and reactions over objects designed by the user. Thousands of companies around the world benefit from the use of CFD software as a main part of their design phases in their product development. It uses the finite-volume method to solve the governing Navier-Stokes equations for a fluid which are derived from the conservation mass equation (1), the conservation of momentum (2) and the conservation of energy (3) equations [6].
(1)
(2)
(3)
The difficulty arises from the fact that the conservation of mass, momentum and energy are coupled and non-linear set of differential equations making them practically impossible to solve analytically for practical engineering problems. Hence CFD software such as FLUENT is utilized to provide very reasonable approximation upon solving the specified governing equations [2].
Additionally, FLUENT also allows the users to model a range of flows such as incompressible or compressible, inviscid or viscous, laminar or turbulent flow. The advanced solver technology that FLUENT has, provides fast and accurate results through flexible moving and deforming meshes to be able to create optimal designs. Ultimately, FLUENT allows engineers to design, create and analyze a configuration all under one program.
In order to model the object that a user wants to work with, its geometry and mesh must be first created in ANSYS Workbench. Another option is to import the geometry and mesh from Computer Aided Design (CAD) software packages such as Unigraphics, ProE or others. In Workbench, the user creates the object he or she wishes to analyze and Workbench guides the user through very complex metaphysics for fluid flow with drag and drop simplicity. Once the geometry has been created, the user can take advantage of several meshing options that Workbench provides. The user can implement the meshing in the specimen to analyze the structure as they try to analyze fluid flow past/through their object. As seen in Fig.3 below that is a mesh for a jet.
Fig.3 Mesh for flow through a jet
A few different ways of modeling and analyzing fluid flow are through turbulence modeling, k-, and Y+. Turbulence modeling is used to model turbulent flow. Turbulent flows are characterized by large, nearly random fluctuations in velocity and pressure in both space and time. These fluctuations arise from instabilities that eventually are dissipated (into heat) by the action of viscosity. Turbulent flows occur in the opposite limit of high Reynolds numbers. The two approaches to solving the flow equations for turbulent flow flied can be roughly divided into two classes, direct numerical simulations and k- [2]. Direct numerical simulation numerically integrates the Navier-Stokes equations, resolving all of the spatial and temporal fluctuations without resorting to modeling. k-, models Reynolds stress in two turbulent parameters, the turbulent kinetic energy (k) and the turbulent energy dissipation rate defined below by Equations 4 and 5 respectively.
(4)
(5)
The next type of modeling is known as y+. Y+ is a mesh-dependent dimensionless distance that quantifies to what degree the wall layer is resolved. Y+ plus is a non-dimensional parameter defined by Eq. [6] [10].
(6)
where u= which is the friction velocity and Yp is the distance to the wall.
Workbench offers several meshing options, one being structured meshing. In structured meshing the user decides how many user defined shapes they want placed over the object they are analyzing. An example is seen in Fig.4. Structured meshing consists of tetrahedrons and exhibits a clearly pronounced pattern.
Fig.4 Structured mesh for a pipe
The mesh interior to a pipe shown is 100 by 5, meaning 100 elements in the horizontal directions and 5 elements in the vertical direction. Which is an example of structured mesh, however as the geometries increase in complexity it is necessary to adjust the meshing accordingly. However, when dealing with other cases such as flow across an airfoil, it is important to use a different mesh structure. One such structure is a structured “O- grid” around the airfoil.Because of the existence of the layers around the airfoil, it can be ensured the flow gradients are properly captured.
Fig.5“O grid” around an airfoil
Referring to the airfoil grid in Fig.5 it should be noted that FLUENT obtains a solution such that the mass, momentum, energy and other quantities are conserved for each cell. The code of the CFD software solves directly the values of the flow variables at the cell centers and the values at other locations are appropriately interpolated [2]. In other cases, where there is no complicated geometry, but rather there is more flow gradients occurring around a certain area, the user can apply a bias. Applying a bias means concentrating the mesh around a certain area. For example, in the turbulent flow past a cylindrical pipe tutorial a bias is applied because as previously learned when dealing with turbulent cases, there are large gradients near the wall requiring the mesh generated as seen in Fig 6.
Fig.6 Bias Mesh for a turbulent flow
In this bias mesh the farther away from the wall, the meshes seem to go back into the same structured mesh seen before. It is important to mention that at the bottom of this mesh it represents the centerline because since cylinders are radially symmetric we’re only showing the top part of the radius. It is expected for the flow to be less turbulent near the centerline and for that reason the mesh is less biased.
Finally, once the test object has been drawn and meshed in Workbench, FLUENT then allows the user to analyze it in different flow parameters. Another modeling capability FLUENT is capable of using is enhanced wall treatment. When the user chooses to use enhanced wall treatment, they can use this especially for turbulent cases using the k-epsilon model because it analyzes the object closer near the wall region. The initial and boundary conditions can be specified in FLUENT and upon initializing the problem; it can be checked for convergence. If the convergence is not achieved accurate results will not be obtained. Finally, FLUENT provides a wide variety of parameters that can be plotted and analyzed.
The topic of convergence requires further explanation in order for a better understanding to be achieved of the underlying steps, undertaken by FLUENT and other CFD packages, necessary to derive a solution. It has already been mentioned that the FLUENT code utilizes the finite-volume method to solve the governing differential equations to obtain a solution for a particulate problem. For simplicity purposes let us consider the finite-difference method which is in 1D. If the grid has equally-spaced points with being the spacing between successive points, the truncation error is O (). As a result as the number of grid points is increased, and the spacing between successive points is reduced, the error in the numerical solution would decrease. Therefore the obtained numerical solution will closely agree to the exact solution [2]. In FLUENT during the obtainment of convergence the governing equations are solved for a predetermined by the user number of times (iterations). Specifically the magnitude of the average of particulate variable is computed as illustrated in Eq. [7] [2].
(7)